Discovery of a bright transient ultraluminous X-ray source Suzaku J1305-4931 in NGC 4945

Abstract

This paper reports the discovery of a bright X-ray transient source, Suzaku J1305-4913, in the south-west arm of the nearby Seyfert II galaxy NGC 4945. It was detected at a 0.5 -- 10 keV flux of $2.2\times 10^{-12}$ erg cm${}^{-2}$ s${}^{-1}$ during the Suzaku observation conducted on 2006 January 15 -- 17, but was undetectable in a shorter observation on 2005 August 22 --23, with an upper limit of $1.7\times 10^{-14}$ erg cm${}^{-2}$ s${}^{-1}$ (90% confidence level). At a distance of 3.7 Mpc, the bolometric luminosity of the source becomes $L_{\mathrm{bol}}=4.4\times 10^{39}\alpha$ erg s${}^{-1}$, where $\alpha =(\cos 60^{\circ }/\cos i)$ and $i$ is the disk inclination. Therefore, the source is classified into so-called ultraluminous X-ray sources (ULXs). The time-averaged X-ray spectrum of the source is described by a multi-color disk model, with the innermost accretion disk temperature of $T_{\mathrm{in}}=1.69_{-0.05}^{+0.06}$ keV. During the 2006 January observation, it varied by a factor of 2 in intensity, following a clear correlation of $L_{\mathrm{bol}}\propto T_{\mathrm{in}}^4$. It is inferred that the innermost disk radius $R_{\mathrm{in}}$ stayed constant at $R_{\mathrm{in}}=79_{-3.9}^{+4.0}{\alpha }^{1/2}$ km, suggesting the presence of a standard accretion disk. Relating $R_{\mathrm{in}}$ with the last stable orbit around a non-rotating black hole yields a rather low black hole mass, $\sim 9{\alpha }^{1/2}$ solar masses, which would imply that the source is shining at a luminosity of $\sim 3{\alpha }^{1/2}$ times the Eddington limit. These results can be better interpreted by invoking sub-Eddington emission from a rapidly spinning black hole with a mass of 20 -- 130 solar masses.

21 pages, 10 figures, Accepted for PASJ 2nd Suzaku special issue

References in corpus (8)

• Hard X-ray Detector (HXD) on Board Suzaku
• Monte-Carlo Simulator and Ancillary Response Generator of Suzaku XRT/XIS System for Spatially Extended Source Analysis
• In-Orbit Performance of the Hard X-ray Detector on board Suzaku
• Suzaku observations of the hard X-ray variability of MCG-6-30-15: the effects of strong gravity around a Kerr black hole
• XMM-Newton Archival Study of the ULX Population in Nearby Galaxies
• Black hole spin in GRS 1915+105
• Suzaku wide-band X-ray Spectroscopy of the Seyfert 2 AGN in NGC 4945
• Detailed spectral study of an ultra-luminous compact X-ray source M81 X-9 in the disk dominated state

Cited by in corpus (13)

• Ultraluminous X-ray Sources in the Chandra and XMM-Newton Era
• The Nature of the Compton-thick X-ray Reprocessor in NGC 4945
• The variable hard X-ray emission of NGC4945 as observed by NuSTAR
• Suzaku wide-band X-ray Spectroscopy of the Seyfert 2 AGN in NGC 4945
• Spectral Survey of X-Ray Bright Active Galactic Nuclei from the Rossi X-ray Timing Explorer
• Suzaku Observations of M82 X-1 : Detection of a Curved Hard X-ray Spectrum
• XMM-Newton Observations of NGC 247: X-ray Population and a Supersoft Ultraluminous X-ray Source
• Transition to the disk dominant state of a new ultraluminous X-ray source in M82
• A new sample of transient ultraluminous X-ray sources serendipitously discovered by Swift/XRT
• Suzaku investigation into the nature of the nearest ultraluminous X-ray source, M33 X-8
• Spectral transitions of an ultraluminous X-ray source, NGC 2403 Source 3
• Second Epoch ALMA Observations of 321 GHz Water Maser Emission in NGC 4945 and the Circinus Galaxy
• Discovery of a transient X-ray source Suzaku J1305-4930 in NGC 4945